The present invention relates broadly to microwave generators, and in particular to a high power microwave generator apparatus utilizing a relativistic electron beam to generate microwave energy as a consequence of induced virtual cathode oscillation. Microwaves can be generated as direct radiation from electrical sparks across gaps by applying a high electric potential. The spark gap can also be a part of a very-high-frequency oscillating circuit which radiates electromagnetic waves. Microwaves can also be derived from the thermal radiation of warm bodies. However all these sources are unsatisfactory because of the lack of purity of the wave and the low power of the radiation. Some important microwave generators are known as klystrons, magnetrons, and traveling-wave tube oscillators. Their power outputs range from microwatts to thousands of kilowatts, depending upon the type and design of the generator and the operating frequency.